Many species of birds and mammals are attracted towards some of their natural predators, often without overt aggression. The biological function of this predator attraction was studied in a colony of herring gulls and lesser black-backed gulls, with the aid of predator models. More birds flocked above the predator model when it was presented together with a dead gull; also the birds did not land as close to a predator with a dead gull as they did to a predator alone. Having seen a predator with a dead gull had a clear effect on the distance at which birds later avoided that particular predator on the ground when it was presented to them alone. It is suggested that predator-attraction enables animals to collect information about a potential enemy; in this information the experience of conspecifics with the predator is taken into account; subsequent behaviour of the animals depends partly on this information. The increase in the avoidance distance with respect to a predator with a dead bird may be an adaptation to carnivores' habit of surplus killing.